Adaptations of the Sherlock canon are a fairly rich area for creators, as there's a lot of material to draw on, and it's all in the public domain. That means people can use or not use whichever bits they like with no restrictions, and put any additional spin on it they like.
That's what's been done in Genius, an audio drama from the perspective of Sabine Moriarty, the sister of our well-known nemesis. She isn't heavily involved with his nefarious schemes, but does on occasion assist him with code-planning and puzzle-setting, having a mind that matches Sherlock's own, but sitting in its own niche.
(Sherlock, for his part, is no numbers expert in this version, and so relies on a friendly professor to solve the riddles she sets him via Moriarty.)
The "riddle", often the method of choice for plotting Sherlock stories, in which a staggeringly complex array of clues and false information, which both Sherlock and often the reader must solve, is not really the driver here. There is an underlying case, and a crime to be answered as one strand of the plot, but this is much more a story of familial conflict and a bit of romance.
Moriarty, as Sabine's adopted brother, has something of an unhealthy dependency on her. He's controlled her life as much as possible for decades, isolating her from others and trying to prevent her forming any sort of romantic attachment. He's presented here in a manic mode, passionate about success, but also prone to self-pity when useful as a manipulation tool. This is a Moriarty skilled in abusive attachments.
As you might imagine, that causes no small friction when Sabine then takes up a romantic relationship with Sherlock. Their burgeoning relationship is the second strand of the plot, with the third and most central being the slow escaping of Sabine from Moriarty's clutches, told throughout the thirteen episodes with a large dollop of flashback. There's lot of time spent here on their childhood and early adult years.
It's a fairly enthralling tale, certainly gripping enough that it took me the first seven episodes to stand back and make the realisation that you can draw no small comparison between this tale and the stereotypical Self Insert Fic. That may be unfair, but it fits a lot of the hallmarks. A new character, important by relation to a nemesis, extremely capable (by dint of being able to match Sherlock's wit as well as being a sharpshooter), and becoming entangled in a romantic liaison with the lead.
That aside however, it's mostly enjoyable. Perhaps a trifle too long, in that the 3/4s mark began to feel a bit of a slog, but in the end, something I'd recommend to any Sherlock fan.